After logging into a dedicated server, teammates and enemy players are occasionally invisible. When it’s teammates who are invisible, they can still be located by a floating nametag and a healthbar that’s continually empty (when playing as Medic). The players are invisible only while alive; after they die, a corpse becomes visible. This state persists through multiple levels, and only seems to be affected by leaving a server and rejoining.
I’ve confirmed with other players that this isn’t just local; players are sometimes invisible to a single player, and sometimes invisible to multiple players. It seems to occur more often with people who are on my friend’s list, but that may just be my perception. I have, however, joined a server and discovered that the entire enemy team was invisible, which made for some awfully confusing combat… I thought they were just really good at hiding at first.
I haven’t figured out any specific steps to replicate, but I’m seeing several occurrences every day so it must not be terribly uncommon.